This invention relates generally to solid state photodiodes, and in particular relates to avalanche photodiode (APD) device design and fabrication.
Avalanche photodiodes are becoming a popular solid state microfabricated device for illumination detection applications. Avalanche photodiodes that are silicon-based can be microfabricated compatibly with standard CMOS processing. When the p-n junction or p-i-n region of an avalanche photodiode is appropriately reverse-biased, illumination of the APD results in photon absorption that generates electron-hole pairs at a region of high electric field in the vicinity of the reverse bias. This electron-hole pair generation produces an electrical signal corresponding to detection of the illumination.
There are two general modes of operation of APDs. In a first mode, known as the linear mode, the reverse bias voltage of the APD is held below the breakdown voltage characteristic of the APD. Under this condition, each photon absorbed at the APD produces on average a finite number of electron-hole pairs, resulting in a characteristic avalanche gain factor that is typically on the order of tens or hundreds. The average photocurrent produced by a linear mode APD is strictly proportional to the incident photon flux.
Although quite adequate for many applications, the signal noise typically associated with linear mode APD operation can be unacceptable for some applications. The linear mode avalanche gain factor is statistically variable, resulting in so-called multiplication noise, which gets progressively worse as the gain factor is increased by raising the APD reverse bias. Once the multiplication noise dominates the noise of signal readout circuitry, the APD signal-to-noise ratio is generally unacceptable for many applications.
The second mode of APD operation overcomes this noise limitation. In this second mode, known as Geiger mode (GM), the APD reverse bias voltage is held above the breakdown voltage characteristic of the APD. Under this condition, the population of electrons and holes generated by photons collected by the APD high electric field region grows exponentially. As the reverse bias is further increased above the breakdown voltage, the exponential growth factor also increases, but the high electric field is reduced by the corresponding growth of avalanche current, given a resistive high field region as is conventional. Ultimately a steady state condition is reached in which charge generation and current flow are balanced, producing a stable APD avalanche current level and a well-defined avalanche turn-on transient time, typically on the order of tens of picoseconds. The avalanche current initiated by a single photon absorbed by a GM-APD therefore produces a well-defined, resistance-limited signal.
Thus, unlike linear mode APD operation, in GM-APD operation, multiplication noise does not apply. The total electron-hole pairs produced by a stable GM-APD avalanche event is fixed by external circuitry, not by statistics of the electron-hole pair generation process. Therefore, unlike linear mode operation, GM-APD operation is not limited by noise; the GM signal can be detected with quantum-limited sensitivity and digitized with effectively noiseless signal readout. This noiseless signal characteristic is particularly well-suited for many advanced illumination detection applications.
Operated correctly, the GM-APD device enables illumination detection to produce optical intensity information on the scale of individual photons. The GM avalanche current pulse produced by the absorption of one photon is indistinguishable from that produced by the absorption of many photons during a given detection period, due to a finite diode quench and reset period. Therefore, GM-APD operation is best directed to detection of single photon events per period. GM-APD operation is accordingly suitable for many important applications, e.g., for low and ultra-low light level imaging applications. The noiseless generation of a fast illumination detection signal is also particularly advantageous for enabling photon detection-based electronic triggering systems. Thus a wide range of systems are particularly well addressed by the sensitive time-of-arrival and/or incremental-count signals that can be produced based on GM-APD avalanche events.
To enable GM-APD operation as opposed to or in addition to linear mode APD operation, an APD structure typically is provided with a designated high electric field region central to the device structure.
Conventionally, this high field region is defined by device features that tailor the electric field profile of the device, in operation, to limit the extent of the high field region. By well-defining the high field region, electric field breakdown at edges of device structures can be eliminated or minimized. In addition, the dark current, i.e., the current produced by the APD under non-illumination conditions, can generally be minimized. Due to the single-photon scale of GM-APD illumination detection and the sensitivity typical of GM-APD applications, such minimization of dark current is generally considered critical for practical applications.
In order to prevent edge breakdown effects, conventional GM-APD structures generally include features that dramatically reduce the fill factor of the device. Specifically, the fraction of the GM-APD structure that is available for photon absorption is generally quite limited by the device features and corresponding field profile employed for control of the GM-APD electric field. As a result, the sensitivity of a conventional GM-APD device can be too low for many important applications. To compensate for this limitation, optical and holographic systems have been employed to concentrate incident light to an intended GM-APD high field device region, in an effort to reclaim absorption efficiency that is lost due to low fill factor. But such compensation systems cannot always be employed. Many low light level applications, such as photon counting applications, require low f numbers and/or small pixel dimensions. In such cases traditional back-illuminated GM-APD structures cannot accommodate an optical system and thus sensitivity cannot be improved with optical focusing techniques. Without an optical compensation system, the reduced GM-APD sensitivity is unacceptable for many important applications.